Asymmetrical 4-crystal monochromator

Abstract: An X-ray analysis apparatus comprises a dispersive system of crystals for monochromatizing an incoming beam in a diffractometer or for analysing an X-ray beam in an X-ray spectrometer. The system of crystals comprises crystals whose crystal lattice planes do not extend parallel to effectively reflective crystal surfaces. As a result, a substantially higher effective radiation intensity can be obtained, for example notably for (220) crystal faces in germanium.

Description: The invention relates to a crystal monochromator for use in an X-ray analysis apparatus, the monochromator consisting of a plurality of germanium monocrystals, the reflecting crystal face each of which does not extend parallel to the diffractive crystal lattice planes in the crystal but encloses a selected angle relative to the (220) lattice planes in the crystal. The invention also relates to an X-ray analysis apparatus provided with such monochromator. A crystal monochromator of this kind is known from a publication in NUCLEAR INSTRUMENTS AND METHODS, Vol.152, 1978, Amsterdam NL, pages 161-166, entitled "DESIGN OF HIGH RESOLUTION X-RAY OPTICAL SYSTEM USING DYNAMICAL DIFFRACTION FOR SYNCHROTRON RADIATION", by K.Kohra et.al. The phenomenon that the reflecting crystal faces used do not extend parallel to the crystal lattice planes is referred to as asymmetry in the context of the relevant technical field. The apparatus as described in said publication is provided with a channel-cut monochromator for use in synchrotron X-radiation consisting of two germanium crystals in an asymmetrical arrangement. In said article it is described that the angle between crystal faces and crystal lattice planes (denoted as α in said publication) can have a value of 14 degrees, which arrangement offers the advantage of providing a higher reflectivity than the conventional monochromators. Because the reflecting crystal faces in the known monochromator do not extend parallel to the crystal lattice planes in the crystals, a relatively large acceptance angle is realised for an X-ray beam to be monochromatized. As a result, for analysis in an X-ray diffractometer an effective X-ray beam with a substantially higher radiation intensity can be generated and a higher detection efficiency can be realised in the X-ray spectrometer. For some kinds of analysis techniques one wishes a relatively high resolution which requires a relatively high monochromatisation of the X-rays. Such high resolution cannot be obtained by using the known 2-crystal monochromator. It could be possible to use a known 4-crystal monochromator for obtaining the required high resolution, but such known monochromators have the drawback of providing a low radiation intensity. For specific applications, for example examination of thin layers, be they imperfect or epitaxial layers and the like, the comparatively low radiation intensity of the known 4-crystal monochromators may become objectionable. Using synchrotron X-radiation this loss of intensity often is not a severe problem because a synchrotron can provide a very high intensity. However, increasing the radiation intensity by using a high-intensity radiation source makes the apparatus expensive and substantially limits the service life of the radiation source. In using a conventional X-ray tube the loss of intensity can be an important drawback. It is an object of the invention to provide an X-ray monochromator providing the choice between operation with a comparatively high resolution and a comparatively high radiation intensity. To achieve this, the X-ray monochromator of the kind set forth in accordance with the invention is characterized in that said plurality is embodied as a 4 and the selected angle between crystal face and crystal lattice planes, is an angle in the range 15° to 23°. By using a 4-crystal monochromator the desired high resolution is obtained. Choosing said angle between the indicated values offers the advantage that having obtained the crystal cut in such a way it is possible to obtain 220-reflections as well as 440-reflections from the crystal, merely by changing the angle between the X-ray beam and the reflecting surfaces of the crystal. These two reflection modes constitute the second and fourth order reflection of the actual crystal lattice plane having Miller-indices (110). Using the 440-reflection one obtains a relatively high resolution; using the 220-reflection one obtains a relatively high radiation intensity. It has been found that if the angle between crystal faces and crystal lattice planes has a value between 15° and 23°, an unexpected increase of intensity for the 220-reflection is obtained. Such a monochromator produces an effective X-ray beam having an intensity which is approximately x times higher than that of the known symmetrical monochromator. Calculations and measurements have demonstrated that x=4 for 15°. Calculations have also demonstrated that x=15 for 20.6°. In order to realise a monochromator which can be fully exchanged, the angle is chosen so that the reflecting crystal faces, measured in the diffraction direction, are large enough to accept the entire incident beam. On the other hand, the value of the angle can also adapted to a desired effective beam intensity for specific examinations. In US 4,928,294 an X-ray analysis apparatus is described comprising a dispersive element in the form of an asymmetric crystal monochromator, consisting of only one monochromator crystal. From this document it is not known to use germanium crystals with diffraction at (220) crystal lattice planes. It is also not known from this document to use any angle between crystal faces and crystal lattice planes between 15° and 23°. In US 4,567,605 an X-ray analysis apparatus is described comprising a dispersive element in the form of a symmetrical 4-crystal monochromator with germanium crystals. From this document it is not known to use any angle between crystal faces and crystal lattice planes between 15° and 23°. In a publication in Nuclear instruments and Methods Research A, Vol.A303, No.3, 15 June 1991, Amsterdam NL, pages 503-514, entitled "Materials science with SR using X-ray imaging spatial resolution/source size" an X-ray analysis apparatus is described comprising a dispersive element in the form of an asymmetric crystal monochromator, consisting of two monochromator crystals. From this document it is not known to use germanium crystals with diffraction at (220) crystal lattice planes. It is also not known from this document to use any angle between crystal faces and crystal lattice planes between 15° and 23°. In a publication in Review of Scientific Instruments Vol.60, No.7, 7 July 1989, New York US, pages 2373-2375, entitled "Dynamical X-ray diffraction from a perfect crystal under grazing incidence conditions" an X-ray analysis apparatus is described comprising a dispersive element in the form of an asymmetric crystal monochromator, consisting of only one germanium monochromator crystal with diffraction at (220) crystal lattice planes. From this document it is not known to use any angle between crystal faces and crystal lattice planes between 15° and 23°. The monochromator carrier may be constructed so that different measurement modes can be selected by rotation of the crystal pairs, for example an asymmetrical (220) position for high intensity and a (440) position for high resolution. However, upon changing over from one measurement mode to the other in this manner it may occur that no detection of a reflection can be observed. This is because a range of zero intensity is traversed during rotation of the crystal pairs. In the case of a small alignment error ( i.e . the angles between the X-ray beam and the reflecting crystal faces deviate slightly from the prescribed value), no reflection will occur any more for any angular rotation. Alignment of the experimental arrangement then becomes very difficult. Therefore, a preferred embodiment of the X-ray analysis apparatus the apparatus comprises a monochromator carrier which is constructed to position in a beam path of an analyzing X-ray beam alternately a first monochromator which is oriented in the (440) crystal lattice plane position and a second monochromator which is oriented in the (220) crystal lattice plane position. Thus the monochromator holder is constructed as a changer system whereby several monochromators can be alternately positioned in the beam path. Because rotation of the crystal pairs is thus avoided, the alignment problem no longer occurs. The monochromator carrier in the form of a changer may comprise asymmetrical crystals as well as symmetrical crystals with a (220) position as well as a (440) position for the crystals, so that crystal rotation is no longer necessary. It should be noted that in US 4,567,605 mentioned above it is described that diffraction from (220) lattice planes as well as from (440) lattice planes can be used. However, from this document it is not known to use a monochromator holder which is constructed as a changer system whereby several monochromators can be alternately positioned in the beam path. Even though the present description often refers to a monochromator for the sake of clarity, the use of the invention is by no means restricted to what is customarily referred to as a monochromator in an X-ray analysis apparatus. An asymmetrically ground crystal system can also be used as an analyzer in an apparatus of this kind (as claimed in claim 4). This is because incoming radiation, now already diffracted from a specimen to be examined, is also discriminated therein in respect of wavelength and/or direction. It may again be advantageous to sacrifice a part of the resolution for a gain in radiation intensity. Some preferred embodiments of the invention will be described in detail hereinafter with reference to the drawing. Therein: Fig. 1 shows an X-ray diffraction apparatus comprising a 4-crystal monochromator as known in the art; Fig. 2 shows diagrammatically a symmetrical monochromator and an asymmetrical monochromator. Fig. 1 shows a known X-ray analysis apparatus, known from US 4,567,605 mentioned above. The apparatus is provided with an X-ray source 1, a monochromator 3, a goniometer 5 and a detector 7 which are only diagrammatically shown. The X-ray source 1 comprises an anode 14 which is accommodated in a housing 10 provided with a radiation window 12, which anode consists of, for example copper, chromium, scandium or another customary anode material. An electron beam generates an X-ray beam 15 in the anode. The monochromator comprises two crystal pairs 18 and 20 with crystals 21, 23, 25 and 27. In the crystal pair 18 reflecting crystal faces 22 and 24 serve as active crystal faces. Similarly, in the crystal pair 20 reflecting crystal faces 26 and 28 act as active crystal faces. The first crystal pair can be arranged so as to be rotatable about an axis 30 extending perpendicularly to the plane of drawing, and the second crystal pair can be arranged similarly so as to be rotatable about an axis 32. The reflecting faces 22, 24 and 26, 28 remain mutually parallel in any rotary position. Preferably, the crystals have, for each pair, a U-shape cut from a single monocrystal, the connecting portion of the U being used, for example for mounting the crystals. The inner faces of the limbs of the U then form the active reflecting crystal faces. After cutting and possibly grinding or polishing, a surface layer has been removed from these surfaces, for example by etching, in order to remove material in which stresses may have developed due to mechanical working. The carrier plate 34 for the monochromator has a comparatively rigid construction so that, for example its lower side can be used to support j mechanical components, for example for the crystal orientation motions, without risking deformation of the plate. In the present embodiment, the length of one of the crystals of each of the crystal pairs is reduced so that more freedom is obtained in respect of a beam path. The attractive property of the 4-crystal monochromator as regards the angle of aperture for the incoming beam enables the X-ray source, i.e. actually a target spot on the anode 14, to be situated at a minimum distance from the first crystal pair, which minimum distance is determined by the construction of the source. An attractive intensity is thus achieved already for the ultimate analyzing X-ray beam 35. In the known apparatus of Fig. 1 (see also Fig. 2a) the first crystal pair 18 is rotatable about the axis 30 of a shaft on which a first friction wheel 40 which is situated beneath the mounting plate is mounted so as to engage a second friction wheel 42 which is mounted on the shaft with the axis 32 about which the second crystal pair 20 is rotatable. However, the two crystal pairs may alternatively be mutually independently adjustable or the adjustment can be performed by means of a drive motor with, for example programmed settings adapted to the anode material to be used or to specimens to be analyzed. The crystals are preferably made of germanium having active reflecting faces which extend parallel to the (440) crystal faces of a germanium monocrystal which is relatively free from dislocations. By diffraction from the (440) crystal faces an extremely well monochromatized beam having, for example a relative wavelength width of 2.3 x 10 -5 , a divergence of, for example 5 arc seconds, width of 2.3 x 10 -5 , a divergence of, for example 5 arc seconds, and an intensity of up to, for example 3 x 10 4 quants per second per cm 2 can be formed. Such a sharply defined beam enables measurement of errors in lattice spacings of up to 1 to 10 5 can be measured and high-precision absolute crystal lattice measurements can also be performed thereby. The monochromatization of the X-ray beam is realized in the monochromator by the central two reflections, i.e. the reflections from the crystal faces 24 and 28. The two reflections from the reflecting faces 22 and 26 do influence the beam parameters, but they guide the beam 35 in the desired direction coincident with the prolongation of the incoming beam 15. Wavelength adjustment is achieved by rotating the two crystal pairs in mutually opposite directions; during this motion, therefore, the position of the emergent beam 35 does not change. An intensity which is, for example 30 times higher can be achieved by utilizing reflections from (220) crystal faces, in which case a larger spread in wavelength and a larger divergence occur. The monochromator is non-rotatably connected to the goniometer 5 in which a specimen 46 to be analyzed is accommodated in a specimen holder 44. For the detection of radiation emerging from the specimen 46 there is provided a detector 7 which is rotatable along a goniometer circle 48 in known manner. The detector enables measurements to be made throughout a larger angular range and for different orientations of the specimen. For exact determination of the position and possible repositioning of the specimen, the goniometer may include an optical encoder which is not shown in the drawing. Fig. 2b shows an example of an asymmetrical system of crystals in accordance with the invention, compared with a similar symmetrical system as shown in Fig. 2a, comprising notably germanium crystals with (440) and (220) lattice planes, respectively. Fig. 2a shows the symmetrical system comprising crystals 21, 23, 25 and 27 in which the lattice planes extend parallel to reflecting crystal faces 22, 24, 26 and 28, respectively. Fig. 2b shows an asymmetrical crystal system in which the lattice planes are chosen to extend parallel to the outwards facing reflecting faces 40, 42, 44 and 46 of the crystals 23, 21, 27 and 25, respectively; however, the inwards facing reflecting crystal faces 22, 24, 26 and 28 no longer extend parallel to the lattice planes in this Figure. Each crystal exhibits (220) as well as (440) lattice planes; in the upper crystal pairs of the Figs. 2a and 2b the (440) lattice planes are used, whereas in the lower crystal pairs of the Figs. 2a and 2b the (220) lattice planes are used. An incoming X-ray beam 15 emerges from the crystal system as a beam 35 which is collinear with the incident beam in all situations. A comparison of the beam diameter of the Figs. 2a and 2b already demonstrates that the difference between the symmetrical and the non-symmetrical system is comparatively small for the (440) crystal planes, whereas it is substantial for the (220) crystal planes. The same holds for the resolution.

Claims: A crystal monochromator for use in an X-ray analysis apparatus, the monochromator consisting of a plurality of germanium monocrystals, the reflecting crystal faces of each of which does not extend parallel to the diffractive crystal lattice planes in the crystal but encloses a selected angle relative to the (220) lattice planes in the crystal, characterized in that said plurality is 4 and the selected angle between crystal face and crystal lattice planes is an angle in the range 15° to 23°. An X-ray analysis apparatus for analyzing a specimen comprising an X-ray source, at least one monochromator, a specimen carrier and an X-ray detection system characterized in that the at least one monochromator is embodied as defined in Claim 1. An X-ray analysis apparatus as claimed in Claim 2, comprising a further monochromator which is embodied as defined in Claim 1 and comprising a monochromator carrier which is constructed to position in a beam path of an analyzing X-ray beam alternately the first mentioned monochromator which is oriented in the (220) crystal lattice plane position and the further monochromator which is oriented in the (440) crystal lattice plane position. A crystal analyzer for use in an X-ray analysis apparatus, the analyzer consisting of a plurality of germanium monocrystals, the reflecting crystal face of each of which does not extend parallel to the diffractive crystal lattice planes in the crystal but encloses a selected angle relative to the (220) lattice planes in the crystal, characterized in that said plurality is 4 and the selected angle between crystal face and crystal lattice planes is an angle in the range 15° to 23°.

Inventors: VAN DER SLUIS PAUL, VAN DER SLUIS, PAUL, VAN DER SLUIS, PAUL, C/O INT.OCTROOIBUREAU B.V.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1994
Main classification: G01N23
IPC codes: G21K1, G01N23